LARAMIE - More of Wyoming's evergreens are turning brown and gray amid an onslaught of bark beetles, and forest managers worry that despite a concerted effort to stem the invasion, the devastation and accompanying wildfire danger will only increase.

Dead and dying trees are being removed, living stands are being sprayed with insecticides, and other chemicals are being applied to discourage bug clustering.

But forest officials fear they are fighting a losing battle. There are simply too many beetles, and they have already wiped out hundreds of thousands of acres of trees across Wyoming.

"It's an extremely serious problem for us," said Jim Myers, a silviculturist for the Medicine Bow National Forest in southeast Wyoming. The mounting dead timber could fuel larger and more intense forest fires.

"The potential there is incredible," he said.

Years of drought and mild winters have created perfect conditions for a beetle explosion.

"It's extensive. It's from border to border. All of our mountain ranges have it," said Terry DeLay, forester for the Medicine Bow's Brush Creek-Hayden District.

Portions of the Shoshone National Forest east of Yellowstone National Park have a 70-percent mortality rate, officials said. Other wooded areas are also under siege.

The drought that has gripped Wyoming for six years has caused the overall health of trees to deteriorate, making timber more susceptible to damage from beetles and other pests. With less moisture, trees are also less able to produce sap, a key defense mechanism because it helps expel the invaders.

Ailments like blister rust have also taken a toll, said Chris Thomas, a forester for the Bighorn National Forest in north-central Wyoming.

"A weak tree - it's like a two-dollar buffet. It's all-you-can-eat for these beetles," he said.

At the Medicine Bow National Forest's headquarters in Laramie, aerial photographs from this year compared to pictures taken two years ago show dramatic growth in beetle-killed stands.

The beetles and the fungus they carry choke off the flow of nutrients within trees as the insects tunnel through the bark. Trees that start losing their greenness were likely attacked the previous summer, Thomas said.

"By the time the needles have turned brown, the beetles have already flown and they're on to the next trees," he said.

Bark beetles are endemic to the West and have periodically invaded forests for thousands of years, foresters say. Severely cold winters, with spells of 30-below temperatures, kill most of the bugs, but winters have been warmer and drier lately.

Single weather events can also play a role. In 1997, strong winds blew down stands of trees along the Wyoming-Colorado border and created perfect breeding grounds.

Logging is one option. Foresters typically select trees - dead, dying or potentially a target - to be removed by private companies. That allows increased air circulation among remaining stands, which disperses chemicals called pheromones emitted by beetles to attract other beetles.

Proceeds from those timber sales are used for restoration; younger trees tend to be more resilient.

More expensive options include insecticides and synthetic pheromones that tell beetles to move on. Those alternatives are mainly used at campgrounds where human activity is greatest.

But those efforts have had limited success.

One of the most popular campgrounds in the Medicine Bow, Silver Lake, has been closed for two years due to danger of falling trees. Disease and a spruce beetle infestation have left as many as 500 trees at moderate to high risk of falling.

Managers were left with no choice but to close the campground, recreation planner Mary Sanderson said.

"It is a liability issue as well as plumb common sense," she said.

Every tree in the campground has been mapped and its potential to cause damage evaluated. The most hazardous ones must be removed before the campground can be reopened, a task which might not be completed until the summer of 2007.

Forest managers are trying to get the word out that they're doing the best they can, given budget constraints, but that a number of trees will die, and that wildfires might follow.

"The beetle epidemic is a natural occurrence, but it's just been exacerbated by the drought," Medicine Bow forest spokeswoman Liz Moncrief said. "We're trying our best to save as much of the forest as we can, but we probably have to anticipate that we're going to lose quite a lot of trees in the meantime."

Capital bureau reporter Robert W. Black can be reached at (307) 632-1244 or robert.black@casperstartribune.net.